Polaris and the 57 navigational stars:
How sailors find their way by the sky
On a clear night at anchor in Pollença Bay in Mallorca, Polaris stands about 40° above the northern horizon. It was there last night and it will be there tomorrow, in the same place, while every other star in the northern sky turns slowly around it. For most of the history of ocean sailing, that one fixed point was the navigator’s best friend.
Polaris will tell you your latitude from a single sight. But it cannot tell you your longitude. For a full position you need other stars, and the Nautical Almanac lists 57 of them for exactly that job. Below is how both work, in plain terms, followed by the bright stars worth learning first and the complete list.
Polaris, the star that stays put
Polaris sits less than a degree from the north celestial pole, the point in the sky directly above the Earth’s North Pole. As the Earth turns, the other northern stars sweep out wide circles around the pole. Polaris traces a circle so small you will never notice it move.
It is not especially bright. Dozens of stars outshine it, and first-timers often look straight past it expecting something dazzling. The dependable way in is the Plough. Find the two stars at the outer edge of the bowl, Merak and Dubhe, and follow the line through them about five times the gap between them. The first star of any note you reach is Polaris, at the tip of the Little Bear’s tail.
Latitude from a single sight
The height of Polaris above the horizon is your latitude. At the North Pole it stands directly overhead at 90°. At the equator it sits on the horizon at 0°. In Palma, at 39.6°N, it hangs just under 40° up.
At sea the navigator measures that angle with a sextant, then applies the standard corrections for instrument error, height of eye and atmospheric refraction. Because Polaris is a fraction off the pole, the Nautical Almanac carries its own Polaris tables. Three small corrections, labelled a0, a1 and a2, allow for where the star sits on its tiny circle at the moment of the sight. The working is short:
Latitude = observed altitude − 1° + a0 + a1 + a2
You need a clear horizon, a rough idea of the time and a star you can find in seconds. No other sight in celestial navigation is this quick, which is why it has been a favourite at evening twilight for as long as sextants have existed.
Why Polaris cannot give you longitude
Polaris always lies close to due north. A sight of it gives a position line running east to west across the chart. You know how far north you are, but you could be anywhere along that line.
Longitude depends on time. The Earth turns through 15° every hour, so a place 15° further west sees the same sky an hour later. To turn a star sight into longitude you need a star that changes its position as the hours pass, and an accurate clock to tell you when you took the sight. Polaris barely moves, so it carries almost no time information. It also drops below the horizon south of the equator.
That is why the almanac treats Polaris separately and leaves it off the list of navigational stars. A Polaris line can still be crossed with a sight of another star to give a fix. On its own it is a latitude sight and nothing more.
How the 57 stars give a full position
At any moment every star stands directly above one point on the Earth’s surface, known as its geographic position. As the planet turns, that point races westward at 15° of longitude an hour. The Nautical Almanac tabulates where each navigational star’s geographic position will be for every hour of the year, measured from the Greenwich meridian.
Measure a star’s height above the horizon and you have, in effect, drawn a circle on the globe around that point. Everyone standing on the circle sees the star at exactly the same height. The circle is usually thousands of miles across, so the navigator only ever works with a short piece of it near the boat.
Turning that into a line on the chart is the job of sight reduction. You start from an estimated position. Tables such as HO 229, or HO 249 Volume 1 for star sights, give the altitude and bearing the star should have from there. Compare that with the altitude you measured. The difference, called the intercept, tells you how many miles to move towards or away from the star along its bearing, and your position line runs at right angles to it. The French naval officer Marcq Saint-Hilaire published the method in 1875 and navigators still use it today.
One star gives a line. Two give a fix. Three give a small triangle, known as a cocked hat, and a check that nothing has gone wrong. Choose stars spread around the horizon, ideally about 120° apart, so the lines cross cleanly.
Twilight and the clock
Star sights work in a narrow window. The stars have to be visible and the horizon still sharp enough to measure against, which happens only in twilight at dawn and dusk. Navigators plan the evening round in advance, working out which stars will appear where so the sextant is pointing the right way when they do.
Then there is the clock. Every four seconds of error in the time of a sight moves the fix by one minute of longitude, which is one nautical mile at the equator. A clock a minute out puts you 15 miles from where you think you are. That arithmetic defeated navigators for centuries, until John Harrison’s marine timekeeper H4 lost only about five seconds on an 81-day voyage to Jamaica in 1761 and 1762. Today a phone synced to GPS, or a radio time signal, does the same job.
Why these 57 stars?
The list was chosen so that wherever a vessel is, and whatever the season, several bright stars are well placed above the horizon at twilight. Brightness counts at sea, because you are spotting them against a sky that is not yet fully dark. Nineteen of the twenty brightest stars in the night sky are on the list. The only one missing is Mimosa, the second-brightest star of the Southern Cross.
The stars are numbered 1 to 57 in order of sidereal hour angle, their position measured westward around the sky, so Alpheratz in Andromeda is No. 1 and Markab in Pegasus is No. 57. For anyone who wants more, a longer table at the back of the almanac covers 173 stars.
The bright stars to learn first
You do not need all 57 to start. These are the ones that sailors in Europe and North America see most often, grouped by the evenings when they are best placed.
Winter
Start with Orion, which straddles the celestial equator and is visible from every ocean. Four of its stars are navigational stars: Rigel, Betelgeuse, Bellatrix and Alnilam, the middle star of the belt. Follow the belt down and to the left and you reach Sirius, the brightest star in the sky. Around Orion, Capella, Aldebaran, Procyon and Pollux complete the Winter Hexagon, a ring of six navigational stars in one patch of sky.
Spring
Use the Plough again. A line from Megrez through Phecda, the two bowl stars nearest the handle, leads away from Polaris to Regulus at the foot of Leo. Follow the curve of the handle the other way and you arc to Arcturus, the brightest star north of the celestial equator, then speed on to Spica in Virgo.
Summer
Vega, Deneb and Altair form the Summer Triangle, high overhead on warm nights and the easiest bright pattern to pick out from a Mediterranean anchorage. Low in the south sits Antares, the red heart of Scorpius, with Shaula marking the scorpion’s sting.
Autumn
Cassiopeia’s W sits opposite the Plough across Polaris and never sets north of about 35°N. Schedar is its brightest star. Below it lies the Square of Pegasus, with Alpheratz and Markab at opposite corners, and to the east Mirfak marks Perseus.
Further south
Head for the Caribbean and the Southern Cross appears low over the southern horizon on spring evenings. Acrux and Gacrux are navigational stars, and two more, Rigil Kentaurus and Hadar, act as the Pointers that lead the eye to the Cross. The southern sky has no pole star, so the long axis of the Cross does that job instead.
All 57 navigational stars
Numbers are the Nautical Almanac star numbers. The equatorial stars lie between 30°N and 30°S and are split by sidereal hour angle: 180° to 360° for the eastern group and 0° to 180° for the western.
Northern stars (north of 30°N)
| No. | Star | Constellation |
|---|---|---|
| 3 | Schedar | Cassiopeia |
| 9 | Mirfak | Perseus |
| 12 | Capella | Auriga |
| 27 | Dubhe | Ursa Major |
| 32 | Alioth | Ursa Major |
| 34 | Alkaid | Ursa Major |
| 40 | Kochab | Ursa Minor |
| 47 | Eltanin | Draco |
| 49 | Vega | Lyra |
| 53 | Deneb | Cygnus |
Equatorial stars, eastern hemisphere
| No. | Star | Constellation |
|---|---|---|
| 1 | Alpheratz | Andromeda |
| 4 | Diphda | Cetus |
| 6 | Hamal | Aries |
| 8 | Menkar | Cetus |
| 10 | Aldebaran | Taurus |
| 11 | Rigel | Orion |
| 13 | Bellatrix | Orion |
| 14 | Elnath | Taurus |
| 15 | Alnilam | Orion |
| 16 | Betelgeuse | Orion |
| 18 | Sirius | Canis Major |
| 19 | Adhara | Canis Major |
| 20 | Procyon | Canis Minor |
| 21 | Pollux | Gemini |
| 25 | Alphard | Hydra |
| 26 | Regulus | Leo |
| 28 | Denebola | Leo |
Equatorial stars, western hemisphere
| No. | Star | Constellation |
|---|---|---|
| 29 | Gienah | Corvus |
| 33 | Spica | Virgo |
| 37 | Arcturus | Boötes |
| 39 | Zubenelgenubi | Libra |
| 41 | Alphecca | Corona Borealis |
| 42 | Antares | Scorpius |
| 44 | Sabik | Ophiuchus |
| 46 | Rasalhague | Ophiuchus |
| 50 | Nunki | Sagittarius |
| 51 | Altair | Aquila |
| 54 | Enif | Pegasus |
| 56 | Fomalhaut | Piscis Austrinus |
| 57 | Markab | Pegasus |
Southern stars (south of 30°S)
| No. | Star | Constellation |
|---|---|---|
| 2 | Ankaa | Phoenix |
| 5 | Achernar | Eridanus |
| 7 | Acamar | Eridanus |
| 17 | Canopus | Carina |
| 22 | Avior | Carina |
| 23 | Suhail | Vela |
| 24 | Miaplacidus | Carina |
| 30 | Acrux | Crux |
| 31 | Gacrux | Crux |
| 35 | Hadar | Centaurus |
| 36 | Menkent | Centaurus |
| 38 | Rigil Kentaurus | Centaurus |
| 43 | Atria | Triangulum Australe |
| 45 | Shaula | Scorpius |
| 48 | Kaus Australis | Sagittarius |
| 52 | Peacock | Pavo |
| 55 | Al Na’ir | Grus |
Getting started
You do not need a sextant to begin. Find Polaris, then add one navigational star a week. A free planetarium app such as Stellarium will confirm what you are looking at until the shapes become familiar.
When you are ready to measure, a plastic sextant costs a fraction of a metal one and is accurate enough to practise latitude by Polaris from the cockpit. The RYA Yachtmaster Ocean course covers the full method, as does US Sailing’s celestial navigation course.
GPS jamming in the Baltic and the eastern Mediterranean has made a backup that relies on nothing but a clock, a book of tables and the sky look a lot less old-fashioned. Start with Polaris tonight.